The capacity of leukocytes to move between blood and tissues or organs, interact with diverse types of other cells within the body, and adjust behaviour and morphology upon environmental changes, are all crucial features for securing the survival of an organism as a whole. As such, an ability to image motile leukocytes in 3 dimensions in situ in tissues and organs over time (the 4th dimension) in a living organism presents a unique and powerful tool that allows for real-time investigation of their function and behaviour. This approach of “seeing” cells in a live animal is called intravital imaging or in vivo microscopy. Although intravital imaging is considered a new tool in biomedical research, in reality, its roots originated in the 19th century. By the end of the 20th century, in an era of fluorescent and confocal microscopy, the technique was substantially enhanced embracing not only new microscopic technologies but also the development of new surgical approaches which expanded the repertoire of tissues and organs that can be imaged. In this talk, I will present the latest advances in research on leukocytes that were made possible with the application of intravital microscopy.